Abstract: Complex karst often restricts human engineering construction.In order to avoid the adverse geological problems related to karst, how to detect and evaluate the karst features more accurately is one of the difficult problems in karst exploration.The borehole ultrasonic imaging technology was utilized to accurately depict the karst phenomenon revealed from borehole, and based on the borehole line karst rate, the concept of 'borehole wall karst rate' was proposed to extend the one-dimensional space expression of karst rate to the two-dimensional space expression, which provided more refined basic data for engineering construction in the karst area.The example shows that the ultrasonic imaging technology could make up for the drilling coring difficulty in karst area, and could display the surface characteristics of borehole wall with high resolution in the whole section, so as to achieve the purpose of detecting karst phenomenon with high precision and evaluating karst characteristics with fine quantitative analysis.The ultrasonic imaging technique to calculate the borehole wall karst rate is not only possible to analyze the variation or stratification of karst development with depth, but also to analyze the characteristics of karst development in different directions of the stratum.